fix(tooling): correct Zig install guidance

[GenericJam]

Summary

• derive the required Zig version from MobDev’s root .tool-versions
• reject mismatched or broken Zig installs before invoking Android build.zig
• make mix mob.doctor report exact, 0.15.x, other-nightly, and failed-version-command states
• replace unretrievable historical-nightly download guidance with exact mise commands
• document why mob.adopt does not destructively rewrite an existing project’s toolchain file

Verification

• mise exec zig@0.17.0-dev.269+ebff43698 -- mix test (2161 passed, 10 excluded)
• mix format --check-formatted
• mise exec zig@0.17.0-dev.269+ebff43698 -- mix compile --warnings-as-errors
• mix credo --strict
• mix erlfmt --check priv/android/crypto.erl
• exact installed toolchain reports {:ok, "0.17.0-dev.269+ebff43698"}
• mix mob.security_scan --strict (dependency/source scans clean; fails only on 6 pre-existing cached-runtime manifest-drift findings)
• git diff --check

Bead: app-840

Companion generator PR: GenericJam/mob_new#41

[GenericJam]

Adversarially reviewed. Approve.

All live guidance sites corrected and routed through the new Toolchain module (deploy hint + doctor, with exact/0.15.x/other-nightly/broken states); every remaining '0.15' in the repo is Zigler-package history or changelog, verified by grep. Gates: 2161/2161 tests, format, credo --strict, warnings-as-errors clean. Empirical: with zig stripped from PATH the printed guidance shows the exact mise commands, and 'mise install zig@0.17.0-dev.269+ebff43698' was proven installable against a fresh MISE_DATA_DIR (notable: mise ls-remote doesn't list dev nightlies but the backend fetches them). Dropping asdf guidance is right — asdf genuinely can't retrieve historical nightlies.

One deliberate behavior change worth naming loudly (will be flagged in the release notes): zig_build_plan now hard-fails ANY non-exact zig, including previously-working legacy setups with app-owned build.zig — the exact-pin philosophy applied at preflight. This converts silent ABI-mismatch build weirdness into an actionable message with install commands. Nit for a follow-up: doctor severity reads missing→warn but mismatch→fail — a wrong zig now reads worse than no zig.

Heads-up for the queue: #44 and #43 both touch native_build.ex and its test — they'll need rebases after this. Merging.